Drawing Number: 21067
Revision: A
PIEZOELECTRIC CHARGE MODE PRESSURE SENSOR GENERAL OPERATION MANUAL
9
With the product R
i
C
s
chosen to be > than R
f
C
f
, the
system discharge time constant is simply R
f
C
f
(seconds).
The feedback parameters of the charge amplifier
establish the low frequency characteristics of the system,
unaffected by the degraded input resistance parameters
of the test sensor and/or cable.
The solution to most low-frequency response limitations
lies in the selection of a high-quality coupling capacitor
of sufficient size (capacitance) to allow a drastic
reduction in insulation resistance of the sensor/cable,
with little or no performance degradation. The PCB
Model 072A coupling capacitor can solve many such
problems; while optional, it is recommended for use
with many high-temperature systems. It consists of a 10
000 pF high-quality polystyrene capacitor in a housing
convenient for threading directly onto most hardline
cable assemblies. See Figures 5.1 (system placement)
and 6.3 (coupling capacitor drawing). The Model 072A
is configured with a 10-32 micro-connector at each end,
one end with the female jack and the other end with a
male plug.
6.5
Other Precautions
Always remember to keep the OPR-GND switch on the
charge amplifier in the GND position while connecting
or disconnecting sensors, cable, or capacitor to the input
connector. Stray or accumulated electrostatic charges
may build to the point that they may saturate or even
damage the input circuitry of the charge amplifier.
Operate the charge amplifier in the SHORT time
constant while the sensor is subject to elevated or
changing temperatures. If it is not necessary to procure
data during the transition from room temperature to
operating temperature, place the OPR-GND switch in
the GND position to keep spurious, thermally generated
charges grounded.
It is prudent to momentarily switch to the GND position
even during the measurement period to ensure that
excess charges do not accumulate at the input of the
charge amplifier.
7.0
MAINTENANCE
The only maintenance required on the pressure sensor is
to keep the connector clean. If it is operating in a dirty
environment, protect the cable connections with heat-
shrink tubing or similar material.
In the event that the electrical connection of the pressure
sensor becomes contaminated with dirt or moisture, the
insulation resistance degrades. This may cause a
reduction of sensitivity or excessive drifting when
connected to the charge amplifier. If this happens, brush
off the connector with no-residue solvent or other
approved cleaning solutions. To restore insulation, bake
the sensor in a vacuum oven at 250°F for about four
hours. If the condition persists even after cleaning
and/or baking, please contact a PCB applications
engineer for further assistance. The sensor is
hermetically sealed; it must therefore be returned to the
factory if the aforementioned measures fail to restore
performance.
